Among the various requirements of color filters for use in multi-color liquid-crystal display devices are high degrees of heat resistance, color fastness or light resistance, and clarity or transmittance. The present invention has been completed on the basis of the Applicant's discovery that acryl resins in particular are useful to achieve high degrees of clarity and additionally excellent chemical resistance. The present invention is further based on the finding that, in respect of the heat resistance and color fastness, organic or inorganic pigments significantly excel dyestuffs in general and are for this reason useful for use in a color filter of a liquid-crystal display device. Thus, the present invention contemplates the use of acryl resins and an organic or inorganic pigment to form a color filter having high degrees of heat resistance, color fastness and clarity. The acryl resins to be used may be either in the form of polymers resulting from polymerization of a specific type of acryl monomers or in the form of copolymers resulting from copolymerization of two or more different types of acryl monomers.
A conventional color filter for a liquid-crystal panel generally uses a base material consisting of a naturally occurring, dyeable organic compound such as typically gelatin or casein purified and decomposed into fractions of smaller numbers of molecules (usually of 1.times.10.sup.4 to 5.times.10.sup.4 molecules). An aqueous solution of the dyeable organic compound, which is soluble in water, is applied to the surface of a transparent substrate for each of the colors to be used for the color filter. The layer of the compound thus formed on the surface of the transparent substrate is patterned to form a multiplicity of separate regions. The separate regions of the compound are then dyed with a dyestuff having a particular characteristic color and are thereafter baked to form discrete color filter elements for the particular color on the substrate. These process steps are repeated for the individual colors for use in the display panel until color filter elements arranged in a multiplicity of triads or any desired groups each consisting of a predetermined number of differently colored filter elements are formed on the surface of the substrate.
A color filter fabricated by this known method however has a drawback in that the organic compound to be dyed is a naturally occurring substance and is for this reason not fully acceptable in respect of the resistance to heat. The use of a dyestuff for imparting a color to the base material of the compound results in another drawback of the prior-art color filter which is not acceptable for its relatively low degree of color fastness of light resistance.
Whereas, various research and development efforts have thus far been directed at the use of pigments in substitution for dyestuffs in fabricating a color filter for a liquid-crystal display panel having excellent heat and light resistances. Pigments in general have however been considered to be unsuited for forming color filters of liquid-crystal display panels having uniform transmittance and excellent color fastness. This is particularly by reason of the relatively low degree of dispersibility of pigments in general.
As well known in the art, the layer of the color elements is sandwiched between the transparent substrate and a transparent electrode to activate the liquid crystals. A thin orientation of typically polyimide resin is formed on this transparent electrode and is baked at high temperature. The layer of the color filter elements is required to withstand the temperature transmitted through the transparent electrode from the orientation film.
On the other hand, liquid-crystal display devices are often used outdoors or installed on automotive vehicles and are thus frequently subjected to solar radiation. The color filters for use in such display devices are required to have sufficiently high degrees of light resistance or color fastness.
Another important requirement of a color filter for use in a liquid-crystal display panel is that the filter elements forming the payer of the color filter has clear-cut contours on the transparent substrate. For this purpose, it is desirable to use a photosetting substance as the colored filter composition of a color filter for a multi-color liquid-crystal display panel structure. A chemical composition having a photosetting property is caused to set when exposed to light and is thus useful for defining filter elements with clear-cut contours in a multi-color liquid-crystal display panel.